Limiting the exposure of oxygen-sensitive materials, e.g. food products, meats, beverages, pharmaceuticals, etc., to oxygen exposure provides a means to maintain and enhance the quality and shelf life of the packaged product. For example, packaging a food product in a package capable of minimizing oxygen exposure is a means to maintain the quality of the packaged product over an extended time and to retard spoilage of the product so that the product is maintained in inventory longer without wastage and the need for restocking and replacement.
When a container is formed of a metal or glass body and is provided with a hermetically sealed closure, the permeation of oxygen through the body and the closure is theoretically impossible because of the impermeability of the materials from which the body and closure are formed. Metal cans may reliably prevent oxygen ingress. However, in both instances some oxygen ingress may occur by diffusion through the gasket or the like positioned between the container body and its lid. When a container is formed of a plastic material, such as a bottle, plastic bag, film, tray or lid, the permeation of oxygen through the body becomes an issue of importance. Further, the quality of the packaged material tends to deteriorate over time, in part because of dissolved oxygen typically present in the packaged material at the time it is placed in the packaging container and also in part due to oxygen ingress which occurs during storage. It has long been recognized that when conventional containers of these types are used for the storage of oxygen sensitive materials, the shelf life of the stored materials is limited.
In the food packaging industry, several techniques have been developed to limit oxygen-sensitive packaged materials to oxygen exposure. Such techniques include the use of a barrier material or layer (a material or layer having low permeability to oxygen) as part of the packaging; the inclusion of some means capable of consuming oxygen other than the packaging material (e.g. through the use of sachets and the like having material capable of reacting with oxygen); and the creation of a reduced oxygen environment within the package (e.g. modified atmosphere packaging (MAP) and vacuum packaging).
Although each of the above techniques has its place in the industry, it is well recognized that the inclusion of an oxygen scavenger as part of a packaging article is one of the most desirable means of limiting oxygen exposure.
It is known to include an oxygen scavenger in a sheet material. The oxygen scavenger reacts with oxygen that is trapped in the package or that permeates into the package. For instance, this is described in U.S. Pat. Nos. 4,536,409 and 4,702,966 and the prior art discussed in these references. The inclusion of oxygen scavengers within the cavity of the package is a form of “active packaging”, i.e., the modification of the package to accommodate a means to regulate oxygen exposure. Normally, the modification is in the form of a sachet or the like introduced into the package cavity. Such active packaging devices have the disadvantages of requiring additional packaging operations, potential breakage of the sachet causing contamination of the packaged goods, and uneven or localized scavenging.
Alternately, regulating the exposure to oxygen involves incorporation of an oxygen-scavenging agent directly into the packaging structure itself. For example, oxygen-scavenging agents have been utilized as part of the package element (film, gasket, coating, etc.) rather than by the addition of a separate structure to the package. Such application has been found to provide a more uniform scavenging effect throughout the package and to provide a means of intercepting and scavenging oxygen as it passes through the walls of the package (herein after referred to as “active barrier” application). Incorporation of a scavenger agent are also used to consume oxygen contained in the packaging article either as residual air oxygen in the packaged goods and/or in the void space within the packaging article not occupied by the packaged goods (herein after referred to as “headspace oxygen scavenging” applications). Headspace oxygen scavenging normally entails the removal of large quantities of oxygen from the interior of the package.
Various agents have been proposed as oxygen scavengers. For example, Michael Rooney, in his article “Oxygen Scavenging: A Novel Use of Rubber Photo-Oxidation”, Chemistry and Industry, Mar. 20, 1982, Pg. 197-198, describes the use of ethylenically unsaturated compounds as oxygen scavengers when exposed to light.
Attempts to produce active oxygen scavenging barrier products include the incorporation of inorganic powders and/or salts into a polymer matrix used to form packaging. Incorporation of such powders and/or salts has been found to cause degradation of the transparency and mechanical properties (e.g. tear strength) of the packaging material and cause processing difficulties in the fabrication of the packaging material.
Attempts have been made to produce active oxygen scavenging barrier products in which a polyamide-metal catalyst system capable of scavenging oxygen is incorporated into a polymeric packaging material. This polyamide based system has the disadvantages of incompatibility with thermoplastic polymers normally used in forming flexible packaging materials, reduced flexibility and heat sealability of the resultant packaging material, and degradation of the polymer's physical properties and structure upon reaction with oxygen.
U.S. Pat. No. 5,399,289, incorporated herein by reference in its entirety, teaches the use of ethylenically unsaturated hydrocarbon polymers (e.g. polybutadiene and like), and copolymers and polymer blends thereof formed by free radical polymerization. This reference teaches that the unsaturation should be limited to 0.01 to 10 equivalents per 100 grams of polymer as the adsorption of oxygen by such systems causes fission of the polymer backbone chain. Such polymers, when reacting with oxygen, normally degrade to low molecular weight products via chain scission and the resultant oxidation by-products can cause degradation of the taste, color and odor of the packaged material (e.g. food products). Further, because these polymers are amorphous, packaging compositions formed with conventional semi-crystalline polymer matrixes are difficult to be blended and processed.
While the prior art compounds may effectively scavenge oxygen, they introduce other problems into packaging. For instance, in summary, the prior art teaches the incorporation of compounds which are ethylenically unsaturated but which often cleave as a consequence of the reactions of the oxygen scavenging process. For example, films containing unsaturated compounds, such as squalene or vegetable oils, produce large amounts of volatile aldehydes and ketones upon oxidation. Unfortunately many of the resultant volatile compounds are not maintained within the film structure and find their way into the headspace of the package. Here they have the potential to degrade the taste, color and/or odor of comestible products.
U.S. Pat. No. 6,254,803 discloses polymers having at least one cyclohexenyl group or functionality as being useful as oxygen scavengers. This reference includes the use of condensation polymers formed from tetrahydrophthalic anhydride, the free acid, and the ester or diester derivatives with a diol or polyol reagent. For example, when the cyclohexenyl containing reactant is a free acid, an anhydride or ester group, the reference teaches that diols, e.g. butanediol, may be used as a co-reactant. Alternatively, the condensation polymer may be formed from a tetrahydrobenzyl alcohol or the corresponding amine or other cyclohexenyl amine which is reacted with compounds having a plurality of functional groups selected from carboxylic acid, acid halide, acid anhydride, isocyano or mixtures thereof. The teachings of U.S. Pat. No. 6,254,803 are incorporated herein in its entirety by reference.
Although polymers formed from tetrahydrophthalic anhydride and the like according to U.S. Pat. No. 6,254,803 do not generate large amounts of oxidation fission products during scavenging, they have limited application where a low Tg is necessary (e.g. applications directed to refrigerated headspace oxygen scavenging). For applications that are directed to refrigerated headspace oxygen scavenging, a low Tg is necessary. However, when this requirement is met, the referenced polymers, in addition to having low Tg, exhibit low melting point, high melt flow index, high tack properties and are viscous liquids at ambient temperature conditions. Such polymers are not pelletizable or readily handled, and are difficult to process into films and other packaging articles using conventional processing equipment. They can provide a resultant product that may not be acceptable for packaging applications.
Ideally, a polymeric material useful in an oxygen scavenging composition should exhibit good processing characteristics, be able to be formed into useful packaging materials, have high compatibility with those polymers commonly used to make packaging materials, and not contain or produce by-products which detract from the color, taste, or odor of the packaged product. Further, the resultant oxygen scavenging composition should be active both under ambient and refrigerated temperature conditions for either headspace oxygen scavenging applications or “active barrier” scavenging applications.
The present invention seeks to address the problems associated with the polymers produced according to U.S. Pat. No. 6,254,803, by seeking to provide compositions that 1) act as oxygen scavengers in packaging applications while minimizing the migration of low molecular weight products out of the packaging material containing the compositions and into packaged goods, and/or 2) can be used under both ambient and refrigerated conditions, and/or 3) can be used for refrigerated headspace scavenging applications, and/or 4) when reacted with oxygen, produce very low quantities of scission and oligomeric by-products, and/or 5) can be readily processed using conventional film forming equipment (e.g. extrusion equipment) or coating equipment to provide a film substantially free of defects, and which can be readily handled to provide a finished packaged article.
It has been found that when condensation polymers comprising ethylenic unsaturation as part of a cyclic group further comprises pendent internal or terminal ionomeric groups as fully described herein below, one can achieve an oxygen scavenger composition having the desired combination of properties indicated above.